The TREX1 Pathway in Autoimmune Disease Lupus is a prototype autoimmune disease with variable clinical features and heterogeneous genetic underpinnings. Our goal in this project is to identify the molecular pathway of TREX1-mediated nucleic acid dysfunction in Lupus and in the Lupus-related diseases to advance our knowledge on the etiology of these related complex autoimmune diseases. There is now an established causal relationship between genetic mutations in the TREX1 gene and a spectrum of human autoimmune diseases. TREX1 mutations cause the autoimmune diseases Aicardi-Goutieres syndrome, Cree Encephalitis, Familial Chilblain Lupus, and Retinal Vasculopathy with Cerebral Leukodystrophy and are found in approximately 0.5-2 percent of Lupus patients in different cohorts, representing one of the most common causes of monogenic Lupus identified. We used our biochemical and structural insights of TREX1 biology to develop a genetically precise mouse model by allelic replacement to recapitulate the disease Familial Chilblain Lupus. The long-term goal is to use this mouse model to generate crucial information on the development and function of the mammalian immune system in the context of dysfunctional nucleic acid metabolism with important implications for the etiology of Lupus. This proposal focuses on the TREX1 D18N mouse model to fully understand this inherited human Lupus mutation in the mouse. Our work on TREX1, as the most catalytically robust 3'?5' DNA exonuclease in mammalian cells, has lead us to propose TREX1 as a critical dismantler of DNA polynucleotides preventing inappropriate immune activation. Failure to efficiently dispose of DNA polynucleotides is a key driver of nucleic acid-mediated immune activation and autoimmune disease, as exemplified by the presence of DNA-associated autoantigens in Lupus and Lupus-like disorders. Phenotypic and molecular analyses of the TREX1 D18N mice will reveal the pathologic mechanisms of this dysfunctional exonuclease. The TREX1 D18N mutant allele will allow us to discover the consequences of dominant genetics and contributions of catalytic mutation to nucleic acid-mediated autoimmune disease. The TREX1 D18N animal model of autoimmunity will unmask the mechanisms of systemic immune dysregulation and the important cell-types and tissues of dysfunction, as well as the pathways of immune and gene activation driving disease pathogenesis. Determining these molecular and cellular mechanisms will uncover new opportunities for therapeutic targeting of DNA polynucleotides that initiate inflammation and autoimmunity. This project is likely to reveal novel concepts indicating that the TREX1 enzyme is a key DNA polynucleotide dismantler and that the failure to appropriately dismantle genomic DNA or DNA from other sources is a key driver in nucleic acid mediated immune activation.